Abstract

This study evaluates the postmortem processes acting during the transition from live to dead assemblages in the Ría de Vigo, Spain. The thanatocoenosis component of benthic foraminifera has been studied along the muddy central axis of the ría and compared with previously published data on living taxa. There are some general differences between live and dead assemblages; biological processes might have caused some of the dissimilarities, but transport of allochthonous foraminifera from nearby areas and the destruction of tests likely have caused the remainder. Assemblages located in the inner parts of the ría and those near shallow, coarse-grained substrates include proportions of allochthonous foraminifera introduced by high-energy bottom currents (e.g., Ammonia heccarii and Haynesina germanica in inner settings and Elphidium crispum, Rosalina anomala,and Cihicides spp. in areas close to coarse sediments). Thin-shelled calcareous tests (e.g., Hopkinsina atlantica and the upwelling-related species Bolivinellina translucens, Leptohalysis scottii, Nonionella stella, and Nonionella turgida) are destroyed soon after the death of individuals. The agglutinated tests of Eggerelloides scaher within the first centimeters of substrate might be prone to disaggregation, possibly as the result of organic membrane decomposition by bacterial activities. The hydrodynamics and microenviromental conditions in the Ría de Vigo are likely to cause displacement or destruction of tests. As a result, microfossil assemblages recovered for analysis are unlikely to reflect the in situ living assemblage at the time of deposition. Recognition of allochthonous taxa is therefore an important part of such studies.